1. Field of the Invention
The present invention broadly relates to measuring halation in transparencies, such as aircraft windscreens made of plastic or the like, and, more particularly, is concerned with a method of providing a quantitative measure proportional to the degree of haze or transparency quality degradation.
2. Description of the Prior Art
The requirement for effective birdstrike protection in today's high-performance aircraft has caused a transition from glass canopies and windscreens to laminated or monolithic plastic transparencies. Plastic aircraft transparencies have a life cycle which appears to be limited by the surface qualities of the transparency. Since the hardness of the plastic surface is less than that of glass, the plastic transparencies are much more susceptible to environmental surface damage than the glass transparencies which they replace.
One of the most significant optical changes that occurs during the life cycle of the plastic transparency is an increase in haze or halation in the transparency. As halation increases, light appears to "spread" from its source, causing a disabling glare or significant reduction in contrast of objects seen through the transparency. This disabling glare can lead to flight safety problems as the pilot's view of the external world is restricted.
Halation, the spreading of light beyond its proper boundaries due to internal reflections or light scattering, is evident whenever a bright light source appears in the field of view of the pilot. One source of halation is particles included in the transparency. Another source of halation is the "volume haze" inherent in most aircraft transparencies. A third, and probably most significant, source of halation is the accumulation of scratches on the surfaces of the transparencies. The effect of aging on a transparency is to increase this source of halation. These scratches tend to cause light to be more evenly distributed over the surface of the transparency, resulting in disabling glare or reduction of contrast between a target and its background.
One recent method of measuring haze disclosed in the above cross-referenced patent application uses an intense light source placed on one side of the transparency to be tested, and a camera and annular neutral density filter positioned on the opposite side. The filter is aligned between the light source and camera lens so as to occlude or shade the lens from the direct light rays, whereas light scattered by portions of the transparency outside the periphery of the filter may be recorded on photographic film in the camera. The annular filter contains contrasting rings of varying shades of gray which facilitate standardization of halation measurements made on different transparencies or the same transparency at different periods in its life cycle.
While the above-referenced method has proven to be satisfactory, it does require that equipment be positioned on both sides of the transparency and accurate alignment achieved between the components thereof. Consequently, a need exists for a halation measuring system which is easier to set up for use.